Coating compositions have long been used to provide the surface of articles with certain desired physical characteristics such as color, gloss and durability. In the past, most coating compositions relied upon a liquid carrier which evaporated after the composition was applied. Recently, however, there has been a growing trend toward reducing the Volatile Organic Compounds (VOCs) of coating compositions. One means of achieving this objective has been through the use of dry, solventless systems such as powder coating systems.
In typical powder coating systems, an electrostatic charge is often applied between the application device (e.g., the spray gun) and the item to be painted. This results in the powder coating's attraction and adherence to the desired substrate.
After being applied, the powder coating is cured. Typically, this is achieved by heating the coated substrate to an elevated temperature (e.g., a temperature between 50.degree. C. and 400.degree. C.). During the curing process, the powder particles melt and spread, while the components of the powder coating crosslink. In addition to not emitting any VOCs into the environment during the application or curing processes, such a powder coating system is extremely efficient since there is essentially no waste (i.e., application yield is approximately 100 percent).
Powder coating systems are well known to those skilled in the art. In the coating industry, the term "powder" includes not only materials with a relatively small particle size, but also those with larger sizes, such as granules. Typically, however, the average particle size of most power coating systems are in the range from about 0.1 to about 500 microns; and more commonly, from about 0.5 to about 100 microns.
Powder coating systems have been used commercially to impart high and low gloss Finishes to surfaces. Notwithstanding the aforementioned advantages, certain problems encountered by the coating manufacturers, transporters and applicators have hindered their widespread use and acceptance. For example, although powder coating systems produce attractive finishes with high levels of application efficiency and low levels of VOC emissions, some finishes lack the necessary durability (e.g., weatherability) properties necessary to make them commercially viable. As used herein, the term "durability" refers to a finish's ability to retain its original gloss after being exposed to ultra violet light over an extended period of time. Accordingly, coating manufactures continually strive to formulate powder coating compositions that produce more durable finishes.
Another problem often encountered with conventional powder coating systems is associated with the physical nature of the uncured powder. Specifically, most uncured powder coatings have an average particle size distribution ranging from about 0.5 to about 100 microns. Powders with such a particle size distribution are often unstable. The term "unstable" as used herein refers to the particles of a powder coating having a tendency to clump or "cake" when exposed to small amounts of moisture, even as low as that resulting merely from elevated humidity conditions. To avoid this phenomenon from occurring powder coating formulators, transporters and applicators all have to take special, and often costly, handling precautions. Accordingly coating manufacturers continually strive to formulate powder coating compositions that are more stable (i.e., powder coatings that resist caking)
Other frequently encountered problems are specific to the use of powder coating systems for the production of low gloss finishes. Historically, gloss reduction in powder coating systems has been accomplished by the implementation of a "differential reactivity" method. This method relics upon combinations of chemistries reacting at different rates to create phase separated domains within the film. These separated domains scatter the reflected light which, in turn, contribute to the film's lower gloss readings.
One problem encountered by this method to produce low gloss powders is that such phase separation is often difficult to control. Another problem encountered by this method is that it usually lowers the film's durability proportional to the amount of gloss reduction. Accordingly, a 20% light reflectance film (as measured at a 60.degree. incidence angle) from a powder coating is not only more difficult to reproduce, but also has poorer durability than a 40% light reflectance film.
Another conventional way of producing low gloss films from powder coating systems is to add flatting agents to the formulation. Typically, the more flatting agents employed, the lower the film's gloss. However, the use of flatting agents typically comes at a cost. Specifically, while flatting agents reduce the finish's gloss, they also tend to reduce the finish's durability. Accordingly, coating manufacturers continually strive to formulate low gloss powder coating compositions that produce durable finishes.
Other problems encountered when using powder coating systems pertain to the use of the same to produce consistent low gloss finishes. Specifically, even though conventional powder coating systems can be formulated to produce gloss levels below 30% light reflectance, it has been difficult for such conventional powder coatings to produce low gloss finishes where the percent of light reflectance is consistently within .+-.5% of the light reflectance from the desired level. Accordingly, coating manufactures continually strive to formulate powder coating compositions that produce more consistent low loss finishes.
While coating manufacturers have been able to formulate powder coating systems which resolve some of the aforementioned problems, they continually try to identify a powder coating system that will resolve most, if not all, of those problems. In other words, if a powder coating system is formulated which can, not only produce durable finishes with consistent gloss levels, but also resists caking, this would be considered by those skilled in the art to be a great advancement of the current technology.